THE SUN: MAIN SOURCE OFENERGY FOR LIFE ON EARTH
Photosynthesis• An anabolic, endergonic, carbon dioxide (CO2) requiring process that uses light energy (photons) and water (H2O) to produce organic macromolecules (glucose). SUN photons 6CO2 + 6H2O C6H12O6 + 6O2 glucose
WHAT IS PHOTOSYNTHESIS? Photosynthesis is the process by which organisms convert light energy into chemical energy in the form of reducing power (as NADPH or NADH) and ATP, and use these chemicals to drive carbon dioxide fixation and reduction to produce sugars. 2 + 22 → 2 + 2 + 2 It is estimated that photosynthesis annually fixes ~1011 tons of carbon, which represents the storage of over 1018 kJ of energy. Photosynthesis, over the eons, has produced 2 the in Earth’s atmosphere.
THE BASICS OF PHOTOSYNTHESIS• Almost all plants are photosynthetic autotrophs, as are some bacteria and protists – Autotrophs generate their own organic matter through photosynthesis – Sunlight energy is transformed to energy stored in the form of chemical bonds (c) Euglena (d) Cyanobacteria (b) Kelp(a) Mosses, ferns, andflowering plants
Light Energy Harvested by Plants Other Photosynthetic Autotrophs 6 CO2 + 6 H2O + light energy → C6H12O6 + 6 O2
WHY ARE PLANTS GREEN?Different wavelengths of visible light are seen by the human eye as different colors. Gamma Micro- Radio X-rays UV Infrared rays waves waves Visible light Wavelength (nm)
WHY ARE PLANTS GREEN?Sunlight minus absorbedwavelengths or colorsequals the apparent color ofan object. Transmitted light
WHY ARE PLANTS GREEN? Plant Cells have Green Chloroplasts The thylakoid membrane of the chloroplast is impregnated with photosynthetic pigments (i.e., chlorophylls, carotenoids).
BRIEF HISTORY:PRIESTLEY’S EXPERIMENTFinding that candles burn very well in air inwhich plants had grown a long time, andhaving some reason to think, that there wassomething attending vegetation, whichrestored air that had been injured byrespiration, I thought it was possible that thesame process might also restore the air thathad been injured by the burning of candles.Accordingly, on the 17th of August, 1771,Iput a sprig of mint into a quantity of air, inwhich a wax candle had burned out, andfound that, on the 27th of the same month,another candle burned perfectly well in it. --Joseph Priestley--
BRIEF HISTORY OF INVENTIONS Priestley later discovered oxygen, which he named “dephlogisticated air” Antoine Lavoisier elucidated its role in combustion and respiration. Dutch physician Jan Ingenhousz ,in 1779 demonstrated that the “purifying” power of plants resides in the influence of sunlight on their green parts. In 1782,the Swiss pastor Jean Senebier showed that CO2, which he called “fixed air” , is taken up during photosynthesis. Nicolas-Théodore de Saussure in 1804, found that the combined weights of the organic matter produced by plants and the oxygen they evolve is greater than the weight of the CO2 they consume. He concluded that water, the only other substance he added to his system, was also necessary for photosynthesis. The final ingredient in the overall photosynthetic recipe was established in 1842 by the German physiologist Robert Mayer, one of the formulators of the first law of thermodynamics, who concluded that plants convert light energy to chemical energy.
PLASTIDS major organelles found in the cells of plants and algae. site of manufacture and storage of important chemical compounds used by the cell. contain pigments used in photosynthesis, and the types of pigments present can change or determine the cells color. Plastids are responsible for photosynthesis, storage of products like starch and for the synthesis have the ability to differentiate, or redifferentiate, between these and other forms. All plastids are derived from proplastids (formerly eoplasts, ), which are present in the meristematic regions of the plant. Proplastids and young chloroplasts commonly divide, but more mature chloroplasts also have this capacity.
In plants, plastids may differentiate into several forms,depending upon which function they need to play in thecell. Undifferentiated plastids (proplastids) may developinto any of the following plastids: Chloroplasts: for photosynthesis Chromoplasts: for pigment synthesis and storage Gerontoplasts: control the dismantling of the photosynthetic apparatus during senescence Leucoplasts: for monoterpene synthesis; leucoplasts sometimes differentiate into more specialized plastids: Amyloplasts: for starch storage and detecting gravity Elaioplasts: for storing fat Proteinoplasts: for storing and modifying protein
The location and structure of chloroplasts Chloroplast LEAF CROSS SECTION MESOPHYLL CELL LEAF Mesophyll CHLOROPLAST Intermembrane space Outer membrane Granum Inner membrane Grana Stroma Thylakoid Stroma Thylakoid compartment
CHLOROPLAST The chloroplast is made up of 3 types of membrane: ◦ A smooth outer membrane which is freely permeable to molecules. ◦ A smooth inner membrane which contains many transporters: integral membrane proteins that regulate the passage in an out of the chloroplast of small molecules like sugars proteins synthesized in the cytoplasm of the cell but used within the chloroplast. ◦ A system of thylakoid membranes
Photosynthesis occurs in two distinctphases: 1. The light reactions, which use light energy to generate NADPH and ATP. 2. The dark reactions, actually light- independent reactions, which use NADPH and ATP to drive the synthesis of carbohydrate from C2 and H2O.
Thylakoids The thylakoid membranes enclose a lumen: a system of vesicles (that may all be interconnected). At various places within the chloroplast these are stacked in arrays called grana (resembling a stack of coins). Four types of protein assemblies are embedded in the thylakoid membranes: ◦ Photosystem I which includes chlorophyll and carotenoid molecules ◦ Photosystem II which also contains chlorophyll and carotenoid molecules ◦ Cytochromes b and f ◦ ATP synthase These carry out the light reactions of photosynthesis
StromaThe thylakoid membranes are surroundedby a fluid stroma The stroma contains: ◦ all the enzymes, e.g., RUBISCO, needed to carry out the dark reactions of photosynthesis ◦ A number of identical molecules of DNA, each of which carries the complete chloroplast genome.
LIGHT REACTIONS (NIELHYPOTHESIS) Americanmicrobiologist Van Niel studied photosynthesis in purple sulfur bacteria. The chemical similarity between 2 S and 2 O led van to propose that the general photosynthetic reaction is where 2 A is 2 O in green plants andcyanobacteria and 2 S in photosyntheticsulfur bacteria
photosynthesis is a two-stage process in which light energy is harnessed to oxidize 2 A (the light reactions): and the resulting reducing agent [H] subsequently reduces C2 (the dark reactions):
VALIDITY OF NEIL HYPOTHESIS1. Hill reaction : In 1937,Robert Hill discovered that when isolated chloroplasts that lack CO2 are illuminated in the presence of an artificial electron acceptor such as ferricyanide, O2 is evolved with concomitant reduction of the acceptor [to ferrocyanide]. This demonstrates that CO2 does not participate directly in the O2 -producing reaction. It was discovered eventually that the natural photosynthetic electron acceptor is NADP, whose reduction product, NADPH, is utilized in the dark reactions to reduce CO2 to carbohydrate.2. Radioactive O : In 1941,when the oxygen isotope 18 O became available,Samuel Ruben and Martin Kamen directly demonstrated that the source of the O2 formed in photosynthesis is H2O
ABSORPTION OF LIGHT: CHLOROPHYLL The principal photoreceptor in photosynthesis is chlorophyll. This cyclic is derived biosynthetically from protoporphyrin IX. Has a central metal ion Mg 2+ It has a cyclopentenone ring, Ring V, fused to pyrrole Ring III. Pyrrole Ring IV is partially reduced in chlorophyll a (Chl a) and chlorophyll b (Chl b), the two major chlorophyll varieties in eukaryotes and cyanobacteria, whereas in bacteriochlorophyll a (BChl a) and bacteriochlorophyll b (BChl b), the principal chlorophylls of photosynthetic bacteria, Rings II and IV are partially reduced.
The propionyl side chain of Ring IV is esterified to a tetraisoprenoid alcohol. In Chl a and b as well as in BChl b it is phytol but in BChl a it is either phytol or geranylgeraniol, depending on the bacterial species. In addition, Chl b has a formyl group in place of the methyl substituent to atom C3 of Ring II of Chl a. Similarly, BChl a and BChl b have different substituents to atom C4.
QUANTUM PHYSICS OF LIGHT ABSORPTION Electromagnetic radiation is propagated as discrete quanta (photons) whose energy E is given by Planck’s law: ℎ = ℎ = where h is Planck’s constant (6.626 x 1034 J.s), c is the speed of light (2.998 x 108 m.s-1 in vacuum), is the frequency of the radiation and is its wavelength (visible light ranges in wavelength from 400 to 700 nm). Thus red light with = 680 nm has an energy of 176 kJ.einstein-1 (an einstein is a mole of photons)
Molecules have numerous electronic quantum states of differing energies. As molecules contain more than one nucleus, each of their electronic states has an associated series of vibrational and rotational sub-states that are closely spaced in energy Absorption of light by a molecule usually occurs through the promotion of an electron from its ground state molecular orbital to one of higher energy But, a given molecule can only absorb photons of certain wavelengths because, the energy difference between the two states must exactly match the energy of the absorbed photon (by law of conservation of energy). The peak molar extinction coefficients of the various chlorophylls, 105 M-1cm-1 ,are among the highest known for organic molecules.
An electronically excited molecule can dissipate its excitation energy inmany ways:1. Internal conversion: a common mode of decay in which electronic energy is converted to the kinetic energy of molecular motion, i.e., to heat. process occurs very rapidly, being complete in 10-11 s. Many molecules relax in this manner to their ground states but Chlorophyll molecules usually relax only to their lowest excited states. Therefore, the photosynthetically applicable excitation energy of a chlorophyll molecule that has absorbed a photon in its short wavelength band, which corresponds to its second excited state, is no different than if it had absorbed a photon in its less energetic long wavelength band.2. Fluorescence: electronically excited molecule decays to its ground state by emitting a photon. Process is much more slower than internal conversion and requires ~10-8 s. A fluorescently emitted photon generally has a longer wavelength (lower energy) than that initially absorbed. Fluorescence accounts or the dissipation of only 3 to 6% of the light energy absorbed by living plants. However, chlorophyll in solution, where of course the photosynthetic uptake of this energy cannot occur, has an intense red fluorescence.
3. Exciton transfer: also known as resonance energy transfer an excited molecule directly transfers its excitation energy to nearby unexcited molecules with similar electronic properties process occurs through interactions between the molecular orbitals of the participating molecules in a manner analogous to the interactions between mechanically coupled pendulums of similar frequencies. An exciton (excitation) may be serially transferred between members of a group of molecules or, if their electronic coupling is strong enough, the entire group may act as a single excited “supermolecule.” Exciton transfer is of particular importance in funneling light energy to photosynthetic reaction centers4 . Photooxidation a light-excited donor molecule is oxidized by transferring an electron to an acceptor molecule, which is thereby reduced. process occurs because the transferred electron is less tightly bound to the donor in its excited state than it is in the ground state. In photosynthesis, excited chlorophyll (Chl*) is such a donor. The energy of the absorbed photon is thereby chemically transferred to the photosynthetic reaction system. Photooxidized chlorophyll, Chl +, a cationic free radical, eventually returns to its ground state by oxidizing some other molecule.
Cyclic Photophosphorylation Process for ATP generation associated with some Photosynthetic Bacteria Reaction Center = 700 nm
CYCLIC PHOTOPHOSPHORYLATION In cyclic electron flow, the electron begins in a pigment complex called photosystem I, passes from the primary acceptor to plastoquinone, then to cytochrome b6f (a similar complex to that found in mitochondria), and then to plastocyanin before returning to chlorophyll. This transport chain produces a proton-motive force, pumping H+ ions across the membrane; this produces a concentration gradient that can be used to power ATP synthase during chemiosmosis. This pathway is known as cyclic photophosphorylation, and it produces neither O2 nor NADPH. Unlike non-cyclic photophosphorylation, NADP+ does not accept the electrons, but they are sent back to photosystem I. NADPH is not produced in cyclic photophosphorylation. In bacterial photosynthesis, a single photosystem is used, and therefore is involved in cyclic photophosphorylation. It is favoured in anaerobic conditions and conditions of high irradiance and CO2 compensation point.
Noncyclic Photophosphorylation Photosystem II regains electrons by splitting water, leaving O2 gas as a by-product Primary electron acceptor Primary electron acceptor Photons Energy for synthesis of PHOTOSYSTEM I PHOTOSYSTEM II by chemiosmosis
PLANTS PRODUCE O2 GAS BY SPLITTING H2O The O2 liberated by photosynthesis is made from the oxygen in water (H+ and e-)
Noncyclic Photophosphorylation Noncyclic photophosphorylation, is a two-stage process involving two different chlorophyll photosystems. Being a light reaction, Noncyclic photophosphorylation occurs on thylakoid membranes inside chloroplasts First, a water molecule is broken down into 2H+ + 1/2 O2 + 2e- by a process called photolysis (or light-splitting). The two electrons from the water molecule are kept in photosystem II, while the 2H+ and 1/2O2 are left out for further use. Then a photon is absorbed by chlorophyll pigments on surrounding the reaction core center of the photosystem. The light excites the electrons of each pigment, causing a chain reaction that eventually transfers energy to the core of photosystem II, exciting the two electrons that are transferred to the primary electron acceptor, pheophytin. The deficit of electrons is replenished by taking electrons from another molecule of water. .
The electrons transfer from pheophytin to plastoquinone, then to plastocyanin, providing the energy for hydrogen ions (H+) to be pumped into the thylakoid space. This creates a gradient, making H+ ions flow back into the stroma of the chloroplast, providing the energy for the regeneration of ATP. The still-excited electrons are transferred to a photosystem I complex, which boosts their energy level to a higher level using a second solar photon. The highly excited electrons are transferred to the acceptor molecule, but this time are passed on to an enzyme called Ferredoxin- NADP reductase|NADP+ reductase(FNR) which uses them to catalyse the reaction : NADP+ + 2H+ + 2e- → NADPH + H+ This consumes the H+ ions produced by the splitting of water, leading to a net production of 1/2O2, ATP, and NADPH+H+ with the consumption of solar photons and water. The concentration of NADPH in the chloroplast may help regulate which pathway electrons take through the light reactions. When the chloroplast runs low on ATP for the Calvin cycle, NADPH will accumulate and the plant may shift from noncyclic to cyclic electron flow
Concept of Light Reaction• Two types of photosystems cooperate in the light reactions ATP mill Water-splitting NADPH-producing photosystem photosystem
HOW THE LIGHT REACTIONS GENERATE ATP ANDNADPH? Primary NADP electron acceptor Energy Primary to make 3 electron acceptor 2 Light Light Primary electron acceptor Reaction- 1 center NADPH-producing chlorophyll photosystem Water-splitting photosystem 2 H + 1/2
IN THE LIGHT REACTIONS, ELECTRON TRANSPORT CHAINS GENERATE ATP, NADPH, O2 Two connected photosystems collect photons of light and transfer the energy to chlorophyll electrons The excited electrons are passed from the primary electron acceptor to electron transport chains Their energy ends up in ATP and NADPH
Chemiosmosis powers ATPsynthesis in the light reactions
CHEMIOSMOSIS POWERS ATP SYNTHESIS IN THE LIGHT REACTIONS The electron transport chains are arranged with the photosystems in the thylakoid membranes and pump H+ through that membrane The flow of H+ back through the membrane is harnessed by ATP synthase to make ATP In the stroma, the H+ ions combine with NADP+ to form NADPH
The production of ATP by chemiosmosis in photosynthesisThylakoidcompartment(high H+) Light LightThylakoidmembrane Antenna moleculesStroma ELECTRON TRANSPORT(low H+) CHAIN PHOTOSYSTEM II PHOTOSYSTEM I ATP SYNTHASE